In a paper appearing in the journal Science, researchers from the University of California, Berkeley, Lawrence Berkeley National Laboratory (Berkeley Lab), Monterey Bay Aquarium Research Institute (MBARI) and Rice University describe an experiment that turned 20 km of undersea fibre optic cable into the equivalent of 10,000 seismic stations along the ocean floor. During their four-day experiment in Monterey Bay, they recorded a 3.5 magnitude quake and seismic scattering from underwater fault zones.
Their technique, which they had previously tested with terrestrial fibre optic cables, could provide much-needed data on quakes that occur under the sea, where few seismic stations exist, leaving 70% of Earth’s surface without earthquake detectors.
“There is a huge need for seafloor seismology. Any instrumentation you get out into the ocean, even if it is only for the first 50 km from shore, will be very useful,” observed Nate Lindsey, a UC Berkeley graduate student and lead author of the paper.
Lindsey and Jonathan Ajo-Franklin, a Geophysics Professor at Rice University in Houston and a Faculty Scientist at Berkeley Lab, led the experiment with the assistance of Craig Dawe of MBARI, which owns the fibre optic cable. The cable stretches 52 km offshore to the first seismic station ever placed on the floor of the Pacific Ocean. This was put there 17 years ago by MBARI and Barbara Romanowicz, a UC Berkeley Professor of the Graduate School in the Department of Earth and Planetary Science. A permanent cable to the Monterey Accelerated Research System (MARS) node was laid in 2009, 20 km of which were used in this test while off-line for yearly maintenance.
“This is really a study on the frontier of seismology, the first time anyone has used offshore fibre optic cables for looking at these types of oceanographic signals or for imaging fault structures,” commented Ajo-Franklin. “One of the blank spots in the seismographic network worldwide is in the oceans.”
The ultimate goal of the researchers’ efforts is to use the dense fibre optic networks around the world — probably more than 10 million km in all, on both land and under the sea — as sensitive measures of Earth’s movement, allowing earthquake monitoring in regions that don’t have expensive ground stations.
“The existing seismic network tends to have high-precision instruments, but is relatively sparse, whereas this gives you access to a much denser array,” added Franklin.
The technique the researchers use is Distributed Acoustic Sensing, which employs a photonic device that sends short pulses of laser light down the cable and detects the backscattering created by strain in the cable that is caused by stretching. With interferometry, they can measure the backscatter every 2 m, effectively turning a 20 km cable into 10,000 individual motion sensors.
“These systems are sensitive to changes of nanometres to hundreds of picometres for every metre of length,” according to Ajo-Franklin. “That is a one-part-in-a-billion change.”
“The beauty of fibre optic seismology is that you can use existing telecommunications cables without having to put out 10,000 seismometers,” Lindsey said. “You just walk out to the site and connect the instrument to the end of the fibre. By using these coastal fibre optic cables, we can basically watch the waves we are used to seeing from shore mapped onto the seafloor, and the way these ocean waves couple into the Earth to create seismic waves.”
To make use of the world’s lit fibre optic cables, Lindsey and Ajo-Franklin need to show that they can ping laser pulses through one channel without interfering with other channels in the fibre that carry independent data packets. They’re conducting experiments now with lit fibres, while also planning fibre optic monitoring of seismic events in a geothermal area south of Southern California’s Salton Sea.
This is not the first time fibre optic cables have been used to detect earthquakes in California. Reasearchers from Stanford university laid a three mile, figure of eight loop of optical fibres beneath the Stanford campus as part of the fibre optic seismic observatory in 2017.
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